The invention relates to a spark-gap arrangement encapsulated in a pressure-proof housing, for the purpose of diverting harmful disturbances due to overvoltage, comprising at least two electrodes disposed substantially opposite one another in a housing that is conductive or provided with a conductive coating, between which a discharge space is formed and which are insulated from the housing, according to the precharacterizing clause of claim 1.
Encapsulated lightning guards with no blow-out function but with surface-gap discharge have been known in the state of the art for years and are marketed, e.g., under the protected trade names DEHNbloc or DEHNgap by the firm of DEHN+Sxc3x96HNE Gmbh+Co. KG. Lightning guards on a spark-gap basis for use in low-voltage power-supply systems accordingly include an electrode configuration disposed within a pressure-proof, insulated housing. The pressure-proof housing itself is formed by a metal jacket. By varying the configuration of the electrodes, various response-voltage values can be achieved. The customary dynamic value of the response voltage in known spark-gap lightning guards is about 4 kV. In these solutions according to the state of the art, the response voltage is predetermined by the distance between the electrodes or can be adjusted.
Particularly in applications involving the protection of electronic devices in the area of information technology and data processing, however, it is desirable for the response voltages to be lower, in the region of ca. 1.5 kV. The small electrode separations required, however, cannot be achieved without additional provisions and/or technological complications.
In guards of the blow-out type, which are unencapsulated, it is possible to lower the response voltage by providing an additional trigger electrode. This trigger electrode, in combination with an electrical triggering device comprising a triggering transformer, causes ignition of a subsidiary spark gap with the result that ignition of the main spark gap is initiated immediately thereafter.
The construction of a spark gap with trigger electrode along with associated triggering device will now be explained in detail with reference to FIG. 1. After the gas diverter (GDT) shown there has been ignited, a pulse transformer TR generates an ignition voltage such that it ignites the subsidiary spark gap F1 and, immediately thereafter, the main spark gap F2.
The response of the subsidiary spark gap F1 is thus initiated by ignition of the gas diverter. The response voltage of gas-filled overvoltage diverters can be varied over quite a wide range and is unproblematic for voltages below 1 kV. The quasi-external subsidiary ignition is amplified by the pulse transformer TR to a voltage level that can reliably ignite the above-mentioned subsidiary spark gap F1.
Hence with a known arrangement according to FIG. 1 it is possible to ignite the spark gap with small input voltages, and thus to achieve the desired protection at low voltage levels, e.g. 1.5 or 2 kV.
In order to apply this proposed method to spark gaps in a pressure-proof encapsulation, it would be necessary to solve the structural problem of inserting a high-voltage trigger electrode through the capsule wall in a pressure-tight manner and making electrical contact with said electrode.
However, this can be achieved only with considerable effort during manufacture and hence is associated with higher costs.
It is thus the objective of the invention to disclose a spark-gap arrangement encapsulated in a pressure-proof manner that comprises at least two electrodes disposed substantially opposite one another in a housing that is conductive or has a conductive coating and forming between them a discharge space, such that the arrangement makes possible a low response voltage in the range of xe2x89xa62 kV with no need to employ trigger electrodes known per se, which must extend externally or be electrically connected outside the capsule.
The objective of the invention is achieved with an object having the characteristics given in claim 1, while the subordinate claims present at least useful embodiments and further developments.
Accordingly, the basic idea underlying the invention is that the metallic housing or metal jacket of the pressure-proof encapsulation of the spark-gap arrangement is used indirectly as trigger electrode, so that a triggering and hence a reduced response voltage can be achieved with no additional elaborate meaures. Preferably the electrodes of the spark-gap arrangement are so shaped as to form a cavity that is quasi closed off toward the interior, such that parts of the outer surfaces of the electrodes are situated in the immediate vicinity of the metal jacket of the housing or a metal coating, being separated therefrom by a relatively thin insulating layer. By this means the outer surface of the jacket or a metallic coating applied thereto can be used as trigger electrode.
The embodiment in which the electrodes form a cavity offers a substantial advantage in that the arc that is generated is surrounded entirely by electrode material, which resists burning away, so that the stress imposed on all the other structural parts by the arc can be considerably reduced. As a result, in combination with the electrode arrangement, even very large pulse currents can be reliably controlled in an extremely small space.
In the case in which high-energy ignition pulses between the electrodes are needed to trigger the main spark gap, and there is a risk of damage to the metal jacket of the housing, in the interior of the housing there is provided a coating of materials resistant to burning away, such as tungsten-copper. Triggering elements provided within the housing that are in electrical contact therewith can extend for different distances into the discharge space or arcing chamber and can also subdivide these spaces into geometrically separated regions, which has positive effects with respect to optimizing an ability to extinguish secondary currents, because in this special configuration of the triggering elements it is possible to generate partial arcs arranged in series.
In another embodiment the trigger electrodes provided in the interior of the housing can consist of a conductive plastic, such as polymethylene oxide (POM). The advantage derived here is that the tendency of the trigger electrode to burn away is similar to that of the insulating material present in the arcing chamber. Because of the release of conductive particles from the plastic that results from the action of the arc, the ignition energy that causes flash-over across the spark gap can be further reduced, so that with relatively little ignition energy large flash distances can be achieved.
If a plastic is chosen that gives off gas, in addition the ability of the spark-gap arrangement to extinguish secondary currents can be further improved.
When the material used for the trigger electrode is a relatively high-resistance, conductive plastic, the bulk resistance of the plastic material can be used to optimize the triggering circuit. As a result, an external protective resistor such as is required in the state of the art is no longer needed, and there is no danger that partial currents will flow over the externally provided wiring. That is, the bulk resistance of the plastic prevents an undesired fraction of the secondary current from flowing through an external pulse transformer.
Hence in accordance with the invention it becomes possible to apply a triggering voltage by way of the conductive housing in order to form a subsidiary spark gap in the discharge space, such that by way of the spark generated in the subsidiary gap within the housing the actual main spark gap between the electrodes can be ignited with little energy.
Preferably the insulation disposed between the discharge space and the housing is interrupted to form an opening in the region of the subsidiary spark gap.
Within this opening in the insulation can be disposed an insulating electrode spacer, adjusted to the shape of the housing and preferably annular, which incorporates at least one slit-like aperture or bore for the path of the subsidiary spark gap.
The electrodes enclosing the discharge space, and disposed opposite one another, can have a concave shape so as to produce the desired quasi-closed cavity.
Furthermore, within the opening in the insulation there can be disposed a insert made of a conductive material resistant to burning away, which is in contact with the conductive housing, projects into the discharge space and takes over the function of an internal trigger electrode. This insert then is connected to the conductive inner wall surface of the housing or to the metal housing capsule itself. By way of the projecting insert of, e.g., tungsten-copper material, it is possible both to produce a preliminary ignition and to generate the main arc. If an insert is provided in a multiple arrangement, e.g. in the form of a slotted disc, several main arcs can be created between the insert and the associated electrode, so that the arc voltage is multiplied or increased.
It is further preferred for the main electrodes to have a conical shape such that they point toward one another, the above-mentioned conductive insert being directed toward the opposite cones and serving to form the subsidiary spark gap on one hand but also, at a greater distance from the main electrodes, the main spark-gap or gaps.
The conductive insert can be shaped at its outer boundary like a disk adapted to the shape of the housing, with an opening the inner ends of which are reduced in thickness and aim toward the electrodes, which are preferably conical. By varying the geometrical parameters it is possible to influence the ignition behavior or the maximal arc voltage over a broad range, with no need to alter the basic shape or basic configuration of the spark-gap arrangement. With a standard housing various electrode inserts can be used as well as trigger electrodes of various shapes, so that a specific product can be adapted to suit different cases of application.
The above-mentioned conductive insert, which acts as trigger electrode, can be designed as a ring with several bridges that extend toward its center, so that the distances between the bridges and the ring form at least part of the discharge space.
In another embodiment of the invention there can be disposed within the opening in the insulation an insert made of conductive plastic and extending at least partly into the discharge space, as has been described above.
A design such that the spark-gap arrangement is rotationally symmetrical with respect to the middle long axis and has a mirror-symmetric shape perpendicular to the middle long axis, as viewed in section through the region of the opening in the insulation, leads to structural simplifications and ensures stability of the electrical parameters.
An alternative embodiment of the invention, without departing from the same basic idea, provides that in the region delimited by the opposed electrode surfaces and the inner surface of the housing, apart from a section assigned to the discharge space, a gas-emitting insulation material is disposed. Embedded in or introduced into the insulation material is at least one trigger electrode, which is connected to the conductive housing, i.e. makes electrical contact therewith. If in this case the trigger electrode is asymmetrically disposed in the space between the electrodes, the subsidiary spark gap is formed across the small distance separating one of the electrodes from the trigger electrode, so that after ignition of the subsidiary gap the actual main spark gap between the main electrodes becomes ignited.
Altogether, the spark-gap arrangement presented here succeeds, by employing a pressure-proof, conductive, as a rule metallic encapsulation, in creating an internal trigger electrode by means of which low response voltages can be ensured, for instance in order specifically to protect electronic devices for data processing or information technology, with no need to maintain minimal distances between the main electrodes; given dynamic response-voltage levels of 4 kV, the inter-electrode distances are already in the range 0.3 to 0.5 mm.
The main electrodes, which form or enclose a cavity, provide protection and reduce burning away of the remaining structural components, so that additional improvements are produced with respect to the long-term stability of a spark-gap arrangement thus constructed. The configuration of the inner trigger-electrode elements allows the arc to be fundamentally subdivided or split into several parts, so that the extinguishing properties of the arrangement can be improved.